US patent application No. 2003/0151513 describes a wireless sensor station network. In operation data from the sensor stations is transmitted in multiple hops to a central panel and vice versa. This is because the sensor stations are not able to transmit directly to the central panel. Each sensor station is given its own, fixed route for transmitting and receiving data to and from the central panel. Typically, the data is first sent to another sensor station and from there to another sensor station and so on until a cluster head is reached. Then the data is sent through a network of cluster heads to the central panel.
After installation of the sensor stations at various locations in a sensing area a network topology is dynamically formed. That is, the routes for each of the sensor stations are automatically selected and programmed into the sensor stations. In US patent application No. 2003/0151513 describes a number of methods of dynamically forming the routes from the sensor stations to cluster head units that channel communication from groups of sensor stations. The initiative is taken by the cluster head unit, which transmits link discovery messages, which alert nearby sensor stations about the availability of the cluster head. These sensor stations forward the link discovery messages. Further sensor stations may receive and forward the forwarded link discovery messages and so on. The link discovery packets function to synchronize communication clocks of the cluster head and the sensor stations.
Next the routes for transmitting messages from the sensor stations to the association unit are defined. In one embodiment this is done by transmitting and forwarding route discovery packets from the association unit. The forwarded route discovery packets indicate the number of hops over which they have been forwarded. In this embodiment each sensor stations picks a route by selecting the association unit or sensor station from which it received a route discovery packet over least hops. The sensor stations record an indication of these routes for later use in normal operation. Subsequently, the sensor stations send route registration packets to the association unit to indicate the route that they have selected.
In another embodiment the sensor stations send link registration messages without waiting for route discovery messages. These link registration messages may reach the association unit via each available route. In this embodiment the association unit selects routes for the sensor stations from which it has received link registration messages. After making the selection the association unit sends route definition messages to the sensor stations for recording an indication of these routes for later use in normal operation.
A problem with these techniques is that they require transmission of information via sensor stations before the routing topology has been defined, i.e. when plural routes are still possible. This complicates the (sensor) stations, because they have to support different modes of communication while operating in low-power mode. If forwarding of information is only done using the definite routing topology, a central approach is slow (each station has to wait until its upward station has centrally been assigned a route) and a decentral approach is inefficient (a routing topology with an unnecessary amount of branches has been created)
Another problem that may occur is that the wireless links between nodes are hindered by barriers and that using the same frequency for upward and downward link routes requires synchronization. This complicates the (sensor) station, because they have to support synchronization and powerful RF signals.
A mobile communications protocol is known from an article titled “TBONE: A Mobile-Backbone Protocol for Ad Hoc Wireless Networks” by Izhak Rubin et al. and published in the IEEE Aerospace Conference proceedings 2002 (Piscataway N.J., Mar. 9-16, 2002) page 2727-2740. The mobile protocol of this article uses two types of network: a high power network termed the BNET, or backbone, and a plurality of low power networks called ANETs. In each low power network (ANET) low power nodes communicate under control of a central node of that ANET. Each of the central nodes also functions as backbone node, i.e. as a node in the high power backbone network.
Devices are provided that are capable of operating as backbone nodes. Initially, each of these devices decides whether it will switch to operation as backbone node, dependent on whether it has a higher qualifications to do so than neighboring devices that are capable of operating as backbone nodes. Subsequently, those devices that have switched to operation as backbone nodes receive join requests from remaining nodes that want to become part of a low power network (ANET) centred on a particular backbone node. The low power networks (ANETs) are formed by granting these join requests. Join requests can come from both remaining backbone capable devices that have not decided to operate as backbone devices, and from low power nodes that are not capable of operating as backbone devices at all.
The article also describes the possibility that remaining backbone capable devices (that have not initially decided to operate as backbone devices) can be commanded to become backbone nodes by existing backbone nodes. This happens when a remaining backbone capable device that is part of a low power network (ANET) receives and relays a join request from a requesting node that cannot be accepted by any existing backbone node. When this happens the central backbone node of the low power network (ANET) commands the remaining backbone capable device to switch to operation as a backbone node and to grant the join request of the requesting node. In this case the remaining backbone capable device joins the high power network (BNET), sets up a low power network (ANET) of its own and ceases to use the paths in the low power network (ANET) of the commanding backbone node. The remaining backbone capable device uses the high power network to relay further messages to or from the requesting node. No provision is made for routing through successive previously established low power network (ANET) nodes and association for this form of routing.
This system addresses the use of two types of network links: links in the low power networks and links in the high power networks. Direct communication links (not via other associated nodes) are used in the low power networks. The selection of communication paths in the high power network is not described, but as high power communication is used longer range paths can be used in this network than in the low power networks. As the system is intended to provide communication in a dynamic environment rather than control of devices in the system there is no single central backbone node. The use of two types of network without central unit makes this system complex and inefficient in bandwidth use.